1. Field of the Invention
The present invention relates to a display apparatus and to a driving method for the display apparatus; more particularly, the invention relates to a moving-image/still-image determination circuit for switching the driving method between that for a moving image and that for a still image, particularly in a display apparatus (hereinafter may be referred to as an LCD) such as a lower power consumption type.
2. Description of the Related Art
Generally, there are two types of driving methods for display apparatuses. One is a sequential-scanning driving method (called a "progressive driving method") that sequentially scans all scanning lines in a frame. Another is a reduction driving method (called an "interlaced driving method") that separates the frame into plural fields and reduces the scanning lines when carrying out the scanning of the scanning lines in the field.
In a field of TFT-LCDs, lower power consumption is increasingly demanded. To satisfy such an increasing demand, use of the reduction driving method from among the two methods is effective. However, the display apparatus must display moving images and still images, and when the reduction method is used for such images, especially for the moving image, defective elements such as lags and tailings are caused. For this reason, the progressive (non-reduction) driving method must be used. Therefore, the two driving methods must be switched between that for the moving image and that for the still image. This requires use of a function that quickly determines whether an image to be displayed is a moving image or a still image.
FIG. 9 shows a drawing illustrative of a conventional moving-image/still-image determination method. As shown in the figure, a frame memory 1 and a comparator circuit 2 are arranged in a display apparatus to perform the moving-image/still-image determination. When the moving-image/still-image determination is performed, image signals for an immediately-preceding screen are stored in the frame memory 1 in digital signal form, and the digital signals corresponding to the image signals for the immediately-preceding screen, which are supplied from the frame memory 1, are compared to digital signals corresponding to current signals in the comparator circuit 2. In this case, all digital signals of pixels on the same position of the screen are compared. As a result, when any difference is found between the signals for the immediately-preceding screen and the current signals, the screens are determined by the comparator circuit 2 to be moving screens, whereas when all the signals for the immediately-preceding and the current signals match, the screens are determined by the comparator circuit 2 to be still images.
Conventional display apparatuses having the moving-image/still-image determination function, however, uses a frame memory having a large capacity sufficiently to store signals corresponding to all pixels of one screen. For example, for a VGA (640.times.480 pixels), a frame memory of about 1 megabyte is necessary. When the moving-image/still-image determination is performed in a configuration such as that described above and a frame memory of a large capacity is used, a larger configuration is necessary and higher cost is required for manufacture. Therefore, use of such a large frame memory increases manufacturing cost of the display apparatus and limits the possibility of size reduction.
Furthermore, such a 1-megabyte frame memory alone consumes 1.5 W of power, while power that can be saved by use of the reduction driving method for still images in a VGA display apparatus is about 0.5 W. Therefore, this power saving by use of the large frame invites converse effects.